Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Aqueous Swelling of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in the Presence of Ionic Surfactants Zhefei Yang,† Shouwei Zhang,§ Volodymyr V. Tarabara,‡ and Merlin L. Bruening*,§ †
Department of Chemistry and ‡Department of Civil and Environmental Engineering, Michigan State University, East Lansing, Michigan 48824, United States § Department of Chemical and Biomolecular Engineering and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Superhydrophilic polyzwitterionic brushes resist fouling, but free ions may screen zwitterion charges and alter brush hydration. This work examines the effect of ionic surfactants on polyzwitterionic brush swelling. In situ ellipsometry shows that the swelling of poly[2(methacryloyloxy)ethyldimethyl-(3-sulfopropyl)ammonium hydroxide] (PMEDSAH) brushes depends on surfactant charge and concentration as well as film thickness. Solutions containing ≥6 mM sodium dodecyl sulfate (SDS) increase the swollen thicknesses of PMEDSAH brushes 2- to 9- fold with respect to thicknesses in water, and increases in swelling are especially high (6- to 9- fold) for thin films. Surfactant adsorption likely breaks ionic cross-links in brushes to enhance swelling, and immersion of brushes in 500 mM NaCl also leads to extensive swelling. Fitting of in situ ellipsometry data suggests that highly swollen films consist of a relatively dense, collapsed base layer covered by dilute brushes that contain about 98% water. At 10 or 20 mM surfactant concentrations, dodecyltrimethylammonium bromide (DTAB) yields much smaller increases in swelling than SDS, presumably because of the hydrophobicity of DTAB. Fluxes through PMEDSAH-modified microfiltration membranes are higher with 2 mM DTAB than with 6 mM SDS, consistent with the higher swelling of thin PMEDSAH brushes in the SDS solution.
1. INTRODUCTION Polyelectrolyte and poly(zwitterionic) brushes, dense assemblies of charged macromolecules grafted from/to a surface,1 can be extremely hydrophilic and provide both enthalpic and entropic barriers to nonspecific adsorption of a wide range of analytes.2,3 Thus, a number of studies employed such coatings as antibiofouling materials,4−10 and these brushes may also serve as lubricating films with extremely low friction coefficients in water.11−14 Additionally, several groups derivatized polyelectrolyte brushes with affinity reagents to capture specific proteins.15−17 In certain cases, the properties of specific brushes can change in response to temperature,18,19 pH,20,21 and solvent,22,23 creating “smart” surfaces. However, the swelling of polyelectrolyte brushes may change dramatically in the presence of salts, depending on the composition of the brushes24 and salts25 as well as the ion valences and concentrations.26,27 Small ions can diffuse into polyelectrolyte brushes and interact with charged monomers to cause a phase transition.26 For polyanion/polycation brushes with pH-independent charge densities, Pincus28 and Borisov et al.29 suggest that the swollen thicknesses of brushes in “saltfree” solutions scale linearly with the chain degree of polymerization (N), irrespective of the chain grafting density. © XXXX American Chemical Society
The ions of dissolved salts screen electrostatic interactions and give rise to chain contraction that depends on the salt concentration. Addition of salt also reduces the osmotic pressure due to counterions within the brushes. As a result of the balance of entropy elasticity and osmotic pressure, the swollen thicknesses of polyelectrolyte brushes (D) depend on the concentration of added salt (CS) and the grafting density of chains (σ) according to eq 1. D ∝ NCS−1/3σ 1/3
(1)
This weak power law dependence on salt concentration agrees with experiments,30−33 but reswelling of collapsed brushes occurs at higher salt concentration in some cases.34,35 Polyzwitterionic brushes contain both positive and negative charges, and as with polyanionic or polycationic brushes, the strong affinity of these charged groups for water leads to superhydrophilic films that are attractive as antifouling coatings.36−38 Nevertheless, the strong inter- and intramolecular associations between opposite charges in polyReceived: August 24, 2017 Revised: January 8, 2018
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DOI: 10.1021/acs.macromol.7b01830 Macromolecules XXXX, XXX, XXX−XXX
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Scheme 1. Surface-Initiated Atom Transfer Radical Polymerization of MEDSAH on (a) Au-Coated Wafers and (b) Nylon Membranes
ionic surfactants. We employ surface-initiated atom transfer radical polymerization (SI-ATRP) to grow poly[2-(methacryloyloxy)ethyldimethyl-(3-sulfopropyl)ammonium hydroxide] (PMEDSAH) brushes on both Au-coated Si wafers and porous nylon membranes (Scheme 1). Films on Au-coated wafers facilitate characterization of brush thickness, swelling, and surfactant adsorption, whereas membranes enable examination of the effect of swelling on filtration.
zwitterionic brushes lead to unusual trends in swelling. For example, in contrast to polyanionic or polycationic films, poly(sulfobetaine methacrylate) (PSBMA) brushes show enhanced swelling in aqueous solutions with increasing ionic strength.39−42 Surfactants with an ionic hydrophilic headgroup and a short hydrophobic tail are vital in diverse applications such as protein solubilization,43 emulsification,44 cleaning,45 and fabric softening.46 Thus, this work investigates the effect of ionic surfactants on the swelling of polyzwitterionic brushes in aqueous solutions. The presence of both cationic and anionic groups in the polyzwitterions may lead to a different response to ionic surfactants than in simple polyanionic or polycationic brushes. Previous research shows that surfactants bind to free polyelectrolyte chains47,48 and polyelectrolyte brushes49−51 through electrostatic as well as hydrophobic interactions. Samokhina and co-workers found that spherical polyanionic brushes shrink in the presence of increasing surfactant concentrations, especially with added salt.51 We showed that the permeability of polyanionic brush-modified membranes increases in the presence of cationic surfactants.52 Although some studies examined surfactant interactions with zwitterionic groups,53−59 very few of them focused on surfactant−polyzwitterionic brush interactions,54,57,59 especially for brushes with varying thicknesses. Zhou and co-workers recently showed no effect of surfactants on the lubrication behavior of polyzwitterionic brushes, suggesting no surfactant uptake.59 However, here we show that surfactant sorption is a function of surfactant charge and concentration as well as brush thickness. This research examines the swelling of zwitterionic PSBMA brushes in the presence of both anionic and cationic surfactants. PSBMA is attractive for these studies because among the common zwitterionic polymers poly(sulfobetaine methacrylate), poly(carboxybetaine methacrylate), and poly(phosphobetaine methacrylate), the swelling of PSBMA in both water and salt solutions is better understood,39,41 which may prove helpful in explaining PSBMA swelling in the presence of
2. EXPERIMENTAL SECTION 2.1. Materials. Hydrophilic nylon membranes with nominal 0.45 μm diameter pores were obtained from Sterlitech (25 mm diameter, NY4525100). Si (111) wafers were purchased from University Wafer (Boston, MA) and coated with Au at LGA Thin Films (Santa Clara, CA) by sputtering 200 nm of gold on 20 nm of Cr on Si wafers. [2(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (97%), dopamine hydrochloride (98%), 11-mercapto-1-undecanol (97%), ammonium chloride (≥99.5%), α-bromoisobutyryl bromide (98%, BIBB), 2,2′-bipyridine (≥99%, bpy), and sodium dodecyl sulfate (98.5%, SDS) were obtained from Sigma-Aldrich. Tris(hydroxymethyl)aminomethane hydrochloride (≥99%, Tris hydrochloride) was purchased from Invitrogen, and dodecyltrimethylammonium bromide (99%, DTAB) was acquired from Acros Organics. Bromine (100%) and triethylamine (100%) were purchased from J.T. Baker, whereas copper(I) chloride (≥99.995%), copper(II) chloride dihydrate (≥99.0%), and dimethylformamide (≥99.8%, DMF) were acquired from Jade Scientific. Acetone (99.7%) and ethyl acetate were purchased from Fisher Scientific, and anhydrous dichloromethane, hexane (analytical reagent grade, AR), and chloroform (AR) were obtained from Macron Fine Chemicals. Dichloromethane was used as received, and chloroform, toluene, and triethylamine were dried with molecular sieves (3 Å, Sigma-Aldrich) before use. Aqueous solutions were prepared using deionized water (Milli-Q, 18.2 MΩ·cm). The disulfide initiator, (BrC(CH3)2COO(CH2)11S)2, was synthesized according to literature procedures.60 2.2. Preparation of PMEDSAH Brushes on Au-Coated Wafers and Nylon Membranes. 2.2.1. Initiator Immobilization. Immobilization of the disulfide initiator on Au-coated wafers included cleaning the substrate with ethanol, drying with N2, cleaning with UV/ozone for 15 min, immersion in a 1 mM ethanolic solution of (BrCB
DOI: 10.1021/acs.macromol.7b01830 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules (CH3)2COO(CH2)11S)2 for 24 h, rinsing with ethanol, and drying under a stream of N2. The protocol for immobilizing the initiator on nylon membranes combined two literature procedures.61,62 Nylon membranes were immersed in acetone for 12 h and then dried with N2. Dopamine hydrochloride (2.1 mmol) was dissolved in DMF (20 mL), and this mixture was degassed via three freeze−pump−thaw cycles before transfer into a glovebag and mixing with triethylamine (1.1 mmol). BIBB (1.1 mmol) was then added dropwise to the mixture with gentle stirring, followed by stirring for 3 h. The solution was subsequently well mixed with 100 mL of Tris hydrochloride solution (2.8 M), and the pH was adjusted to 8.5 using NaOH. Clean nylon membranes were immersed in this dopamine solution for 48 h, removed from the glovebag and the solution, and rinsed with ∼100 mL of water and dried with N2, forming polydopamine initiators (PDA-BIBB) on the surface. 2.2.2. Polymerization of MEDSAH. PMEDSAH brushes on Aucoated wafers were prepared using the parameters of a procedure for the synthesis of polyanionic brushes.52 MEDSAH (35 mmol) was dissolved in a DMF/water mixture (1/1 v/v, 30 mL) in a roundbottom flask. The mixture was degassed via four freeze−pump−thaw cycles. The catalyst was prepared by dissolving CuCl (0.016 mmol), CuCl2 (0.016 mmol), and bpy (0.08 mmol) in a DMF/water mixture (1/1 v/v, 40 mL) and degassing via four freeze−pump−thaw cycles. Then 5 mL of the Cu−bpy solution was mixed with the monomer solution in a N2-filled glovebag. The polymerization solution was injected into vials, and wafers or membranes modified with initiators were submerged in these solutions. After various polymerization times, the wafers or membranes were removed from the glovebag, rinsed with ∼100 mL of water, and dried with N2. All membrane samples in this work were prepared with a polymerization time of 24 h. Polymerization times on Au-coated wafers ranged from 0.5 to 8 h. 2.3. Characterization of Modified Au-Coated Wafers and Nylon Membranes. 2.3.1. Fourier Transform Infrared Spectroscopy (FTIR). Au-coated wafers modified with disulfide initiator and PMEDSAH brushes were characterized by reflectance FTIR spectroscopy (Nicolet 6700 FT-IR spectrometer, Thermo Scientific, with a Pike grazing angle (80°) attachment and p-polarization). A UV/ozonecleaned Au-coated wafer served as a background. To examine surfactant adsorption to swollen brushes, brush-modified wafers were transferred directly from water to surfactant solutions without drying. After a 12 h immersion in the surfactant solution, the wafer was rinsed with ∼100 mL of water from a wash bottle for ∼2 min prior to drying with N2 and obtaining a reflectance FTIR spectrum. To determine whether surfactant adsorbs to brushes in solution and can be removed by water rinsing, wafers were not rinsed by water after removal from the surfactant solutions. Instead, the solution on wafers was wiped off with a KimWipe to remove surfactants in solution, followed by IR characterization. A bare Au-coated wafer treated in the same way served as a control. IR data are reported as a pseudo-absorbance, which is the logarithm of the ratio of the reflected intensities from the bare wafer and from the film-coated wafer. 2.3.2. Ellipsometry. Both dry and swollen thicknesses and refractive indices of the polymer brushes on Au-coated wafers were determined using a rotating analyzer ellipsometer (model M-44, J.A. Woollam) with an incident angle of 75°. For swollen films, brush-coated wafers were immersed in solution in a homemade trapezoidal cell with glass windows.63,64 To determine brush thickness and optical constants, the refractive index of the brush was fit using an effective medium approximation (EMA) model, as shown in Figure 1. For samples in air, the EMA model contains a Cauchy material for PMEDSAH brushes along with air-filled voids. Refractive indices in the Cauchy material were described by eq 2
n(λ) = A +
B λ2
Figure 1. Models for PMEDSAH brushes in (a) air and (b) solutions. With solutions, the model employed the minimum number of EMA (effective medium approximation) layers needed to fit the data, with a maximum of three layers. aqueous solutions, an ambient layer representing the solution was set as the top material above the EMA layer(s) (Figure 1b). The refractive index of the ambient solution was taken as that of water at 23 °C,65 but in the case of 0.5 M NaCl we added 0.005 to the refractive index values for water. This addition corresponds to the difference in the refractive index (at 588 nm) between 0.5 M NaCl and water. The fractions of polymers and solutions in the EMA layer(s) were determined by fitting ellipsometric data. To determine the number of EMA layers required to model ellipsometric data for swollen films, we examined the agreement between experimental and fitted Δ and ψ values as well as the polymer volume (per unit area) obtained from fitted models of dry and swollen films. Typically, with the exception of very thin films the polymer volume per unit area in surfactant solutions (swollen thickness multiplied by the polymer volume fraction(s) in the EMA layer(s)) and in air (dry thickness multiplied by the polymer volume fraction in air) differed significantly, so for all but the thinnest films in surfactant solutions we utilized two or three EMA layers in the model. Figure S1 shows calculated polymer volumes per unit area using models with different numbers of layers, and Tables S1−S14 provide fitting results. (Table and Figure captions beginning with S refer to the Supporting Information.) 2.3.3. X-ray Photoelectron Spectroscopy (XPS). XPS was carried out with a PerkinElmer PHI 5600 ESCA system with an Al Kα X-ray source (1486.6 eV) running at a power of 350 W at a takeoff angle of 45°. 2.3.4. Scanning Electron Microscopy (SEM). The surface morphology of membranes was observed with field-emission SEM (JSM-7500F, JEOL). The membranes were mounted on the sample studs with double-sided conductive adhesive tape. A thin layer of osmium (8 nm) was coated on the sample surface prior to the SEM imaging, which was performed at an accelerating voltage of 5 kV. To characterize their cross sections, membranes were frozen in liquid nitrogen and after removal from the nitrogen quickly cracked with a razor blade to create a sharp edge with minimal membrane deformation. The histogram of pore sizes was determined manually using Nano Measurement software (v 1.2.5, Fudan University, Shanghai, China). 2.3.5. Contact Angle Measurements. Contact angles of water or hexadecane on surfaces were estimated using a phone camera and contact angle calculation software (VCA2000, AST Products). In air, this included simply placing a drop of water on dry samples. To determine the surface hydrophilicity in water and surfactant solutions, the samples were immersed in these solutions for 12 h and placed facing downward in a funnel capped with a rubber stopper and filled with water or surfactant solutions. A drop of hexadecane was released from beneath the samples using a syringe (Figure S2). We employed this method because hexadecane droplets tend to float away from needles and surfaces in these solutions. 2.4. Determination of Membrane Permeate Flux and Brush Swelling on Membranes. The specific fluxes (L/(m2 h bar), LMH/ bar) through membranes were determined in dead-end filtration using a 15 mL Amicon cell (Model 8010, Millipore) with a suspended stir bar rotating at 45 rpm. The cell was connected to a stainless steel feed tank (standard 2 gallon pressure vessel, Pope Scientific) (Figure S3),
(2)
where n is the refractive index, λ is the wavelength of incident light in μm, and for the polymer, A (dimensionless) and B (μm2) are constants with values of 1.5 and 0.01, respectively. The fractions of both polymers and air were initially estimated as 50%, and typical fitted air fractions were ∼15%. When samples were immersed in C
DOI: 10.1021/acs.macromol.7b01830 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules and the applied pressure was constant at 0.7 bar. Pure water flux was determined using deionized water as the feed, whereas the permeate flux in surfactant and salt solutions was measured by filling the feed tank with those solutions. Changes in the swelling of PMEDSAH brushes on membranes were estimated via the Hagen−Poiseuille equation66 J mπr 4 = ΔP 8ητ Δx
in part from the insolubility of PMEDSAH in common organic solvents despite full dissolution of the monomer MEDSAH. Thus, polymerization may proceed in a heterogeneous system that gives rise to a broad molecular weight distribution and large thickness variations.70 We employed XPS to characterize the growth of the PDABIBB initiator and zwitterionic PMEDSAH brushes on nonreflective nylon membranes. Figure 3 shows XPS wide-
(3)
where J is the permeate flux (L/(m h)), r is the radius of membrane pores, m is the number of pores per unit area, ΔP is the transmembrane pressure (0.7 bar), η is the dynamic viscosity, which is almost the same for water and the solutions examined in this work,67−69 Δx is the membrane thickness (∼100 μm), and τ is the pore tortuosity. Thus, by comparing water fluxes of modified membranes and bare nylon membranes and assuming constant τ and m, the pore size of modified membranes can be estimated using eq 4 and a value for the radii of pores in the unmodified membrane. 2
⎛ r ⎞4 = ⎜ 1⎟ J2 ⎝ r2 ⎠ J1
(4)
The swollen thickness of brushes is equal to the difference between the pore radius of bare membranes and brush-modified membranes. (This is a crude approximation given the spongy nature of these membranes.) All of the filtration experiments were repeated with two or three membranes.
3. RESULTS AND DISCUSSION 3.1. Brush Synthesis on Au-Coated Wafers and Nylon Membranes. Scheme 1a shows the SI-ATRP strategy we employ to grow polyzwitterionic brushes on Au-coated wafers. The reflective Au surface enables brush characterization with reflectance IR spectroscopy and ellipsometry. IR spectra confirm both the disulfide initiator adsorption (Figure S4) and PMEDSAH brush growth (Figure S5). In particular, the presence of sulfonate stretches at 1218 and 1041 cm−1, a CO stretch at 1731 cm−1, and a C−N stretch at 1486 cm−1 are consistent with the presence of brushes on the surface. Figure 2
Figure 3. XPS wide scan spectra of (a) a bare nylon membrane and membranes modified with (b) PDA-BIBB initiator and (c) PMEDSAH brushes.
scan spectra of the surfaces of a bare nylon membrane, a PDABIBB-coated nylon membrane, and a PMEDSAH-coated nylon membrane. After the polymerization of dopamine-BIBB on the membrane, signals representing Br 3d (76 eV), Br 3p (188 eV), and Br 3s (261.1 eV) appear, which is consistent with the presence of initiating groups on the PDA-BIBB coated surface (Figure 3b). The strong S 2p (172 eV) and S 2s (236 eV) signals detected only on PMEDSAH-coated membranes (Figure 3c) confirm the growth of PMEDSAH brushes. Signals from Br remain after polymerization, suggesting that some initiators are unreacted or some of the chain ends remain active after 48 h of polymerization. SEM images show the differences in morphology for bare as well as PDA-BIBB- and PMEDSAH-modified nylon membranes. Top-view SEM images and histograms of pore sizes (Figure 4) suggest blocking of small pores or reduction in the size of larger pores after coating with PMEDSAH. Crosssectional images show that the membrane remains porous throughout the modification steps (Figure S6), but due to the broad size distribution of interconnected pores in the membrane, we cannot say whether interior pores decrease in size after formation of polymer brushes. 3.2. Brush Swelling and Wettability. This section examines the swelling and wettability of a series of PMEDSAH brushes in deionized water. For water-swollen films, a single EMA layer was sufficient to fit ellipsometric data (see Figure S1). This is consistent with neutron reflectivity data that suggest that in water the polymer density does not vary greatly with distance from the substrate.41 Figure 5 shows the swollen thicknesses of PMEDSAH brushes on Au-coated wafers in
Figure 2. Dry film thicknesses (blue circles) and IR absorbances of sulfonate groups at 1218 cm−1 (red squares) as a function of polymerization time for PMEDSAH brushes grown on Au wafers. Error bars represent the standard deviations of measurements on three different films.
shows that both the peak height for the sulfonate stretch at 1218 cm−1 and the dry ellipsometric thickness of PMEDSAH brushes increase with polymerization time, indicating continuous brush growth. With the exception of the longest polymerization time, the sulfonate-stretch peak height and thickness increase proportionally, giving confidence in the characterization techniques. The relatively large standard deviations in Figure 2 and disagreement in IR and ellipsometry data at the longest polymerization time may suggest a rough surface and some uncontrolled polymerization. This could stem D
DOI: 10.1021/acs.macromol.7b01830 Macromolecules XXXX, XXX, XXX−XXX
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longer chains give more chances for cross-linking and formation of denser networks. After the dry brush thickness surpasses 20 nm, the brush density in water is relatively constant. We characterized surface wettability by measuring hexadecane contact angles on the surfaces of brushes in both air and water. As Figure 6 shows, on PMEDSAH-coated wafers and
Figure 6. Contact angles of hexadecane on bare Au-coated wafers (a, d), PMEDSAH-modified wafers (b, e), and PMEDSAH-modified membranes (c, f). The images were obtained in air (a−c) and water (d−f). Images in water are rotated 180°. Figure 4. SEM top-view images (A, B, C) and histograms of pore sizes (a, b, c) of (A, a) a bare nylon membrane and membranes modified with (B, b) PDA-BIBB initiator and (C, c) PMEDSAH brushes.
membranes, the contact angle of hexadecane increases from ∼10° to 150°−170° after changing the ambient region from air to water, independent of the thicknesses investigated in this work. In most cases, hexadecane will bead up and eventually detach from the surface on both wafers and membranes. In control experiments, bare Au-coated wafers show hexadecane contact angles smaller than 20° in both air and water. These results show that PMEDSAH brushes on both gold and modified nylon become superoleophobic underwater, consistent with previous research.62,73 Huck and co-workers74 investigated the surface wettability of ATRP-synthesized PMEDSAH brushes in air as a function of thickness. The hydrophilic brushes (advancing water contact angle of 10°−20°) became much less hydrophilic (advancing water contact angle of ∼80°) at a critical thickness that depends on the polymerization rate, molecular weight, and grafting density. Brushes with low molecular weights and grafting densities were always hydrophilic, whereas longer chains with a low grafting density were hydrophilic until the chains presumably had a sufficient molecular weight to form interchain associations with neighboring chains and undergo a phase transition. We did not see a similar transition for hexadecane contact angles in water. Aqueous swelling might preclude a phase change, or perhaps the phase change occurs in a dense layer underneath more dilute brushes and, thus, does not affect the contact angle. 3.3. Interactions between Ionic Surfactants and PMEDSAH Brushes. PSBMA brushes are well-known as antibiofouling materials,75−77 and they also show significant adsorption of inorganic salts, which increases brush swelling.39,41 Ionic surfactants contain both charged groups and alkyl backbones and possess properties of both ions and hydrophobic molecules. Thus, surfactants may affect swelling differently than simple salts, so this section examines interactions between PSBMA brushes and ionic surfactants as well as the resulting brush swelling. 3.3.1. Anionic Surfactant (SDS). 3.3.1.1. Effect of PMEDSAH Brush Thickness on Swelling. To study SDS adsorption as a function of PMEDSAH brush thickness, we prepared an extensive series of films with dry thicknesses varying from 2 to 400 nm. As Figure S8 shows, the IR spectra of PMEDSAH-coated wafers before and after immersion in SDS solutions are essentially identical. The only difference is
Figure 5. Water-swollen thicknesses (red squares) and percent swelling (blue circles) of PMEDSAH brushes as a function of their dry thickness. Error bars represent standard deviations of measurements on three films and are not always visible due to their small values. Swollen thicknesses were determined in water after a 12 h immersion.
deionized water as a function of the dry brush thickness. Over most of the range of dry thicknesses, the swollen thickness (D) is a linear function of the dry thickness, with ∼150% swelling, in good agreement with neutron reflectivity data reported by Takahara and co-workers.41 Assuming a constant grafting density, the PMEDSAH brushes obey the D ∝ N rule in eq 1. Although greater than the dry thickness, the swollen thickness is still much less than the fully extended chain length (see below for more discussion on chain molecular weight) because of attractive interactions between sulfobetaine groups and the entropic penalty of chain extension. Previous research shows that the formation of intragroup, interchain, and intrachain associations in polyzwitterionic brushes yields cross-linked networks.39 However, the thinnest brushes (dry thickness
